Various applications require filtering of multiple combined spectral components in signals by selecting one or more of the spectral components while rejecting the other components. One example is band pass filtering where a selected one or more spectral components within a spectral pass band are selected to transmit and spectral components outside the spectral band are rejected. A filter may be tunable, e.g., under a control of a tuning control signal, to change the frequency range of the filtered signal.
Multi-channel radio frequency (RF) switching currently requires large arrays of switches. Such switch arrays are heavy, large, complex, and costly. In space applications, RF switch arrays cannot be reprogrammed easily from the ground, resulting in the use of multiple arrays to provide on-orbit channel agility. For example, the uplink beam switch and channel filter network deployed in a conventional payload are forced to use two large switch arrays in order to provide uplink beam selection prior to channel filtering.
Optical band pass filters are known where optical spectral components within a spectral window transmit through the filter while other spectral components outside the spectral window are rejected. It is known to construct optical band pass filters using optical resonators, which are small electro-optical devices, having diameters on the order of millimeters, formed of curved optical waveguides, for example, a cylinder, a sphere, or a toroid within which light is internally reflected at the inner surface of the optical resonator. Some optical resonators can support resonator modes of light called whispering gallery modes, and thus, are often referred to as whispering gallery mode resonators. Whispering gallery modes occur when light having an evanescent wave component travels via internal reflection around the periphery of the optical resonator. The whispering gallery modes of optical resonators reside close to the surface of the optical resonator, and undergo total internal reflection. The evanescent wave component extends beyond the optical resonator's outer surface and may be coupled into an adjacent optical coupler as long as the optical coupler is located within the extent of the evanescent wave, typically on the order of the light's wavelength.
Many optical resonators which propagate whispering gallery modes of light have extremely low transmission losses, and as a result, have a very high quality factor Q. High Q optical resonators are desirable because the higher the Q, the longer the amount of time the internally reflected light will remain within the optical resonator.
Optical domain filters are able to filter any desired signal including RF, microwave, millimeter, Gigahertz or Terahertz frequency that is modulated as a sideband on an optical carrier. The use of whispering gallery mode resonator technology allows for designing optical domain filters with features of small size and weight, suitable for ground as well as spacecraft applications.
It is desirable to develop a system and method for wavelength locking between the optical source and the optical resonators used to construct an optical filter, wherein a conversion of received signals to an optical domain allows each channel of the filter to select between multiple uplink beams without the need for external RF switch arrays, thereby reducing a size and a weight of the channel filter banks.